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The Pleistocene to upper middle Miocene section was not cored in Hole M0028A, and the hole bottomed in the lower Miocene. Chronology for the lower middle to lower Miocene in Hole M0028A was obtained by integrating on an age-depth diagram (Fig. F23) the following:

  1. Biostratigraphy provided by calcareous nannofossil, planktonic foraminifer, and dinocyst data. Zonal durations are plotted using tables provided in "Paleontology" along with select datum levels (first and last occurrences).

  2. Sr isotopic ages and associated age errors.

  3. Placement of sequence boundaries and other stratal surfaces (Table T14).

  4. The timescale of Berggren et al. (1995; BKSA95).

Surfaces were picked as seismic sequence boundaries in two-way traveltime (Table T14) and converted to depth and tied to cores (Table T14) using a velocity function (see "Stratigraphic correlation" in the "Methods" chapter for discussion of criteria used to define seismic sequence boundaries and the derivation of the time to depth conversion of seismic data). In figures, features are presented as follows:

  • If unsampled by cores, the reflector is indicated with a gray shaded zone.

  • If identified in a core, the surface was plotted as a solid red line for sequence boundaries and a dashed line where uncertain or where there was more than one possible core surface for a given seismic sequence boundary (see "Stratigraphic correlation").

  • A few prominent (but not all) features interpreted as MFSs identified in the cores (see "Stratigraphic correlation") are indicated with a green line.

  • A visual best fit sedimentation rate line was estimated for each sequence and an age error assigned to each. The sedimentation rate is not corrected for postdepositional compaction and represents a minimum sedimentation rate.

The section above sequence boundary m4.5 (247.78 mbsf) is constrained only by one Sr isotope age of 13.8 Ma ± 1.17 m.y. and is assigned to calcareous nannofossil Zones ?NN6 and NN5 and planktonic foraminifer Zone M7/N10. The overlap of G. praemenardii (FO ~14.2 Ma) and G. archeomenardii (LO 13.9 Ma) suggests that the base of this sequence is ~14 Ma. This sequence is older than sequences m3 and m4, which correlate with the two major steps (Mi3 and Mi4) in the middle Miocene oxygen isotopic record (Miller et al., 1998).

Sequence m5 (246.0–268/271 mbsf) is constrained in age by one Sr isotope age estimate of 14.0 Ma ± 1.17 m.y. and is assigned to dinocyst Zone DN5 and nannofossil Zone NN5. Its age is determined largely by superposition as 14–15 Ma. Sequence m5.2 (268/271–313.48 mbsf) is constrained in age by Sr isotope age estimates of 15.8 Ma ± 0.6 m.y. and 16.1 Ma ± 0.6 m.y. to Zone DN4 and Zones NN5 and NN4. The NN4/NN5 zonal boundary provides a firm age constraint (15.6 Ma) within the sequence, with a basal age of ~16 Ma obtained by assuming a high, constant sedimentation rate. Sequence m5.3 (313.48–335.5/342.4 mbsf) is constrained in age by Sr isotope age estimates of 16.7 Ma ± 0.6 m.y. and 16.4 Ma ± 0.6 m.y., Zone NN4, and Zone DN2–DN3. Its age is not well constrained (~16–17 Ma), with a possible age of 16.4–16.8 Ma. Sequence m5.4 (335.5/342.4–?495.2 mbsf) is assigned to Zones NN4, M3/N6 or older, and DN3 (DN2 dinocysts are reworked into this sequence). Sr isotope ages of 16.6 Ma ± 0.6 m.y. and 17.3 Ma ± 0.6 m.y. are concordant with the biostratigraphy, but two ages of 18.8 and 18.3 Ma may suffer from reworking, as noted by the dinocysts. The basal age is 17.3–18.3 Ma, with a best estimate of ~18 Ma. Sequence m5.45 (?495.2–512.3 mbsf) is assigned to Zones NN4 (<18.4 Ma) and DN2–DN3. The sole Sr isotopic age estimate of 18.6 Ma ± 0.6 m.y. is consistent within errors. The sequence boundary is clearly younger than 18.4 Ma but could be as young as ~17 Ma with a best estimate of ~18.2 Ma. Sequence m5.47 (512.3–519.7 mbsf) has little age constraint other than superposition. The assignment to dinocyst Zone DN2 appears too old compared to constraints on the overlying and underlying sequence. Sequence m5.6 (519.7–544.5/546.7 mbsf) is only constrained by the presence of G. praescitula (FO 18.5 Ma) and the presence of Zone NN4 (<18.4 Ma) at the top. Extrapolation of sedimentation rates would yield an age range of ~18.6–19.6 Ma. Sequence m5.7 (544.5–600.3 mbsf) has no constraint other than superposition (18.6 to ~20.5 Ma). Sequence m5.8 (600.3–663.0 mbsf) is assigned to mid-Zone NN2 (~20–21.5 Ma) and Zone DN2. Provisional magnetostratigraphy (see "Paleomagnetism") identifies a thick (>18 m) normal magnetozone in the lower part of this sequence, overlain by a more complicated interval that appears reversed. Given the biostratigraphic constraints (~20–21.5 Ma) that are quite consistent with regional correlations (including Hole M0027A), this normal magnetozone may be Chron C6AN.2n and the reversal below it Chron C6AN.2r, suggesting a basal age of ~21.5 Ma. Sequence m6 was just reached at the bottom of the hole, with an Sr isotope age of 20.7 Ma ± 0.6 m.y.

Sedimentation rates are difficult to estimate in Hole M0028A from the preliminary age constraints. Typical sedimentation rates shown on Figure F23 are 40 m/m.y. Sedimentation rates during deposition of the m5.4 sequence in a position near its greatest thickness were higher: the ~145 m of this sequence was deposited in 1–1.5 m.y. with sedimentation rates of ~100–145 m/m.y. The relatively thick sequences m5.2 and m5.8 (~43 and 51 m, respectively) appear to have been deposited in <1 m.y. with rough sedimentation rates of ~50 and 75 m/m.y., respectively.